Silica gels are used for biochemical and industrial processes, such as in the manufacture of foods, beverages, drugs, and in research, to bond ("bind") or adsorb, or to absorb selected materials. Current important uses include ion exchange chromatography, size exclusion chromatography, selective adsorption of molecules, etc. The two most coveted gel qualities in a silica gel are (1) specific binding to a given molecule, and (2) the best balance of the opposite properties of large pore size and high surface area. Conventional gels have irregular pore structures, so that various size molecules are retained. They also can have either large pores (high P.V.) or many pores (high S.A.), but one can only be increased by reducing the other. There has long been a need for silica gels having high surface area and high pore volume and gels having the ability to specifically bind with a given molecule are also sought. The subject invention provides a gel having high surface area, high pore volume and specificity. Further, the subject gels are able to absorb greater than 200 lbs of oil per 100 lbs of gel. This property makes the subject gels particularly useful in applications requiring a high degree of oil absorption such as in polyol purification, as a flatting pigment in paint, or as a carrier for dl-alpha-tocopheryl acetate (Vitamin E). In addition, the gel of the subject invention has a broad utility beyond the selective adsorption desired and sought in the past.
The subject invention discloses a new type of silica gel. The subject gel is made by modifying the unsuccessful and subsequently abandoned specific adsorbent preparation technology of the 1940's and 1950's, and introducing additional processing steps after gel polymerization. The earlier described gels, discussed hereinbelow, were attempts to create a silica gel that specifically binds to a given molecule. Although there has been a long felt need for such a gel, previous attempts using this older technology to create a gel that specifically binds to a given molecule have failed.
In the past, the manufacture of silica gels was attempted by "molding around" a dye molecule; that is, acidifying an aqueous alkali metal silicate solution containing a dye such as methyl orange or the like, air drying the resulting gel, then washing the gel to remove as much dye as possible. The term "molding around" is used to describe the use of a given molecule as a template for the formation of a silica gel, the gel produced having an increased specific ability to adsorb the template molecule. Attempts to create practical gels by the "molding around" method have failed because appreciable amounts of dye remained in the gel. Accordingly, early hopes of silica gel "molded around" a dye molecule were abandoned as unfeasible. Frank H. Dickey, Specific Adsorption, J. Phys. Chem., 59: 695-707 (1955); Sidney, A. Bernhard, The Preparation of Specific Adsorbents, JACS, 74: 4946-4947 (1952); Linus Pauling, Tailor-Made Compounds Predicted, C. & E. N., 27: 913 (1949)
Related silica gels made by a process similar to old method described above were also reported in a communication to the editor of the Journal of the American Chemical Society. In this communication, experiments for separating optical isomers of camphorsulfonic acid and mandelic acid were described. R. Curti and V. Colombo, Chromatography of Stereoisomers with "Tailor made "Compounds, JACS, 74: 3961 (1952) In these experiments, gels were "molded around" either camphorsulfonic or mandelic acid molecules instead of dye molecules. The results obtained by using these gels in the chromatotographic separation of solutions containing the two acids were a 30% enrichment of 1-camphorsulfonic acid and a 10% enrichment in the best fraction of mandelic acid. The term "enrichment" refers to the relative increase in concentration of a molecule with respect to the molecules from which it is being separated. As this report was in a communication to the editor, and not a peer reviewed article, it is most likely that preliminary results were reported in an attempt to claim credit for the first successful gel "molded around" a molecule. As these results were never again duplicated in the prior art and no subsequent article corresponding to the communication was published, it is reasonable to deduce that no successful gel that is "molded around" a molecule has been produced. In addition, nowhere has the problem of removing residual molecules to which the gel is "molded around" been solved. A compendium including the above work has been prepared by Ralph K. Iler. The Colloid Chemistry of Silica and Silicates, Cornell University Press, pp. 150-152 (1955).
The silica gels of the prior art use "secondary modification" to achieve high surface area, large pore volume or increased specificity. "Secondary modification" refers to treatment of the gel after the gel has been polymerized and washed. Secondary modification typically comprises additional steps such as aging, treating with hot ammonia, etc. These additional steps increase the time required for production and increase costs. By using secondary modification there is normally a tradeoff of surface area for increased pore volume. Accordingly, it is extremely difficult to create a gel having high surface area and high pore volume.
A high surface area silica gel is described in U.S. Pat. No. 3,607,777, issued Sep. 21, 1977 to Winyall. Winyall discloses a surface area of about 650-950 m.sup.2 /g. However, the pore volume is only 0.35-0.8 cm3/g. In addition, no specificity is imparted to the gel.
A process for increasing pore volume of intermediate density silica gels is disclosed in U.S. Pat. No. 3,526,603, issued Sep. 1, 1970 to Acker. This involves a secondary treatment of the formed gel with a hot ammonia solution. Although, two of the gels produced have surface areas and pore volumes within the range of the subject invention, secondary modification, i.e. treatment with hot ammonia, is required to generate these characteristics. By offering a method which does not require secondary modification, the subject invention allows a more economical gel to be produced. Additionally, the gels produced by Acker are not specific to a given molecule.
Increased specificity has been obtained in the prior art by incorporating compounds into the gel, but not by molding a gel around a molecule that acts as a template. An example of a specific gel is described in U.S. Pat. No. 4,169,926, issued Oct. 2, 1979 to McDaniel which describes the addition of a catalyst metal specific to 1-olefin polymerization. The addition of the catalyst of McDaniel is achieved by calcination and as such represents a secondary modification.
A method of producing amorphous silica of controlled oil absorption is described in U.S. Pat. No. 4,312,845, issued Jan. 26, 1982 to Wason. Wason teaches that silica gels having an ability to absorb oil at 60 cm.sup.3 /100g (approximately 54 lbs/100 lbs) or greater, are desired for the production of dentrifice, and flatting pigment in paint, and for use in polyol purification.
Among the advantages of this invention over the prior art is the combination of high pore volume and high surface area. The subject invention also requires no secondary modification to be specific and a such represents a major improvement over the prior art. Further, the subject gel is made by a direct and simple manufacturing procedure which allows a superior product to be produced at a lesser cost.